Hydrogen may be stored and transported as a high pressure gas, as a low-temperature liquid at atmospheric pressure, or in the form of chemicals having hydrogen stably but reversibly bonded thereto. The transportation and storage of hydrogen, especially in gas or liquid form, involve a risk factor that may become extreme, as hydrogen has a broader flammability range than other types of fuels.
The most common transportation method is that of pressurized gas (20-70 MPa) in pipelines and cylinders made of aluminum, steel or carbon-reinforced aluminum for higher pressures.
Nevertheless, this transportation method is affected by the drawback associated with embrittlement of steel caused by the diffusion of atomic hydrogen into the steel.
Such embrittlement may lead to a sudden failure of the cylinder structure, with obviously detrimental consequences.
Since hydrogen liquefies at ambient pressure at a temperature of about 20K, the containers for transportation and storage of liquefied hydrogen must meet extreme thermal insulation requirements.
For this reason, this type of transportation is almost exclusively reserved to specific applications, such as space launchers.
In order to obviate the difficulties of transporting and storing hydrogen, hydrogen-rich chemical compound may be used, such as hydrides, e.g. metal hydrides.
Metal hydrides retain hydrogen due to a reversible reaction with the metal. They are formed and act through absorption and release of hydrogen.
Particularly, the dissociation of the hydrogen molecule allows atomic hydrogen to diffuse into the metal, thereby forming a solid solution. As the concentration of hydrogen increases in the metal, the hydride phase starts to grow, until the metal is entirely in this phase.
This hydrogenation is a reversible process, whereby hydrogen can be released back from the hydride so formed.
Hydrogenation is an exothermic process, i.e. a process that releases large amounts of heat, whereas the release process is endothermic and requires large amounts of heat.
For this reason, transportation and storage of hydrogen using metal hydrides is a safe process.
Prior art metal hydride devices for storage and transportation of hydrogen gas use cylinders designed for storage of high-pressure hydrogen gas, which are filled with metal powders (to form hydrides).
Nevertheless, these commercially available cylinders for storage of high-pressure hydrogen are not suitable to obtain an optimal chemical and kinetic hydrogen absorption and desorption reaction using metal powders because the accumulation of the powders in large-diameter cylinders, like designed and manufactured for high operating pressures and different storage methods, tends to heat to very high temperatures the cylinder during hydrogenation and to overcool during release.
Such considerable heat accumulation and cooling hinder the corresponding processes, which requires the provision of high-performing heat exchangers operating on the cylinder.
In light of the above, the Applicant has suggested the opportunity of optimizing the hydrogen absorption/desorption capacity in metal hydride devices, and particularly the heat exchange between the device and the outside environment.